Protective immunity, whether induced by infection or by vaccination, relies on the generation of memory B &T cells in sufficient quantity, and of sufficient quality. In the case of CD8+ T cells - to be studied herein - these memory cells usually represent the few survivors of a dramatic and rapid cull, T cell contraction, in which 90-95% of virus specific CD8+ T cells (in our model, ~50 million cells) are removed over a period of 1-2 weeks. The process of T cell contraction remains poorly-understood, and the overall goal of this proposal is to better characterize the causes and consequences of T cell contraction. The application has the following 4 Specific Aims: 1. To investigate the role of direct IFN3 signaling in regulating CD8+ T cell contraction. IFN3 is a key cytokine. Its receptor is expressed on almost all somatic cells, allowing it to act both as an antiviral effector molecule, and as an immunomodulatory molecule. We have developed a novel approach that allows us to evaluate the direct effects of IFN3 on T cells during virus infection, and we have found that - contrary to much of the published literature - the effects of IFN3 are strongly positive;T cells that are unable to receive IFN3 signals during virus infection are 100-fold less likely to enter the memory pool. In this aim, we propose a detailed analysis of these direct effects. 2. To determine how indirect effects of IFN3 affect T cell contraction. The widespread expression of the IFN3 receptor means that IFN3 also can affect T cell biology indirectly, via a multitude of paths. Some of these indirect effects will be evaluated. 3. To evaluate antigen persistence after acute virus infection is cleared, determine its effects on the quality and quantity of T cells, and analyze the role of IFN3 in these effects. Antigen contact plays an important part in regulating T cell function, and we have developed a new approach that allows us to identify T cells that have recently encountered authentic viral antigen in vivo. We shall ask how recent antigen contact correlates with a variety of T cell phenotypes (proliferative status, expression of cytokine receptors, etc.). 4. To manipulate T cell contraction, and determine the consequences on the quantity and quality of memory cells. Most studies suggest that T cell contraction is relatively random, but I propose that it may be selective, possibly serving to separate the wheat from the chaff. To investigate this, various approaches will be taken to modify the contraction phase, and the quantity and quality of the surviving T cells will be determined. PUBLIC HEALTH RELEVENCE:T cells are a vital part of the immune response. They help us recover from many viral and bacterial infections, and they are important in vaccine-mediated immunity to infection and disease. When a microbe enters our body, we produce many millions of these T cells to fight the infection but, after the infection has been cleared, the vast majority of the T cells die. In this grant, I investigate how and why the T cells die. I also will attempt to reduce the amount of T cell death, in the hope that the increased number of surviving T cells will give us increased protection against infection and disease.